Conventional papermaking requires the removal of significant amounts of water. The final water removal is typically done by evaporative drying. A conventional papermaking machine has a dryer section with a plurality of drying cylinders. The paper web to be dried is guided in contacting relationship through the plurality of cylinders. The cylinders may be arranged in two or more staggered rows, so that the paper web assumes a serpentine path.
In the art, a single-wire draw may be used, particularly at the beginning of the multi-cylinder dryer. In a single-wire draw, the drying wire is arranged to support the web as it moves from one cylinder to a successive cylinder.
Also, twin-wire draws are known in the art. In a twin-wire draw, the paper web has an open draw when it moves from one cylinder to a successive cylinder. Also known in the art is a Pistol-Grip draw. In a Pistol-Grip draw, the top wire of one cylinder section is wound below the bottom cylinder of the next cylinder section, and the web is supported by the top wire to the next top cylinder.
Examples of papermaking machinery can be found in U.S. Pat. No. 4,888,883, iss. Dec. 26, 1989 to Kerttula; U.S. Pat. No. 5,046,266, iss. Sep. 10, 1991 to Autio; U.S. Pat. No. 5,475,934 iss. Dec. 19, 1995 to Eskelinen et al.; U.S. Pat. No. 5,495,678, iss. Mar. 5, 1996 to Ilmarinen et al.; U.S. Pat. No. 5,535,527, iss. Jul. 16, 1996 to Virta et al.; U.S. Pat. No. 5,537,755, iss. Jul. 23, 1996 to Kotitschke; U.S. Pat. No. 5,539,999, iss. Jul. 30, 1996 to Kuhasalo; U.S. Pat. No. 5,560,123, iss. Oct. 1, 1996 to Eskelinen; U.S. Pat. No. 5,572,801, iss. Nov. 12, 1996 to Ahokas et al.; U.S. Pat. No. 5,666,741, iss. Sep. 16, 1997 to Bubik et al.; and U.S. Pat. No. 6,105,277, iss. Aug. 22, 2000 to Lindberg et al., incorporated herein by reference.
The paper machine clothing generally serves a variety of competing purposes. It can support the web without separation; it should allow adequate permeability for transport of water to be removed from the web and it should provide contact of the web against the drying cylinders, while also contacting the reversing cylinders. Attempts have been made in the art to provide suitable drying fabrics. For example, belt-like material having selected permeabilities are known. The selected permeabilities are provided by varying the spacing of the machine direction yarns, the diameter of the machine direction yarns, or adding chemical treatment in the spaces between the machine direction yarns.
In yet another attempt in the art, the clothing has a controlled void volume. The void volume is controlled by providing a multi-layer fabric, a synthetic, polymeric thermoplastic resin foam may fill the void spaces to control the void volume.
In yet another embodiment, to reverse the adverse effects of over-pressure on the sheet at the outer face of a dryer fabric, void spaces at the fabric-cylinder interface receive boundary air compressed between the paper machine clothing and the cylinder. The void space forming surface may include spaced parallel ribs defining grooves therebetween. The grooves reduce the rate and extent to which boundary air moves into the reducing space between the fabric and cylinder is compressed. This has a corresponding effect on reducing the amount of air forced through the fabric.
In yet another attempt in the art, the dryer fabric includes a plurality of spiral coils extending in the machine direction. Adjacent coils are intermeshed and held together by a hinge yarn. This arrangement is said to reduce occurrences of slack edges. Slack edges in the dryer fabric do not fully press the paper sheet against the cylinder, causing different drying rates to occur in the machine direction. This results in a non-uniform moisture profile across the sheet.
Examples of the foregoing attempts in the art may be found in U.S. Pat. No. 3,867,766, iss. Feb. 25, 1975 to Wagner; U.S. Pat. No. 4,224,372, iss. Sep. 23, 1980 to Romanski; U.S. Pat. No. 4,364,421, iss. Dec. 21, 1982 to Martin; U.S. Pat. No. 4,813,156, iss. Mar. 21, 1989 to Ashworth et al.; and U.S. Pat. No. 4,857,391, iss. Aug. 15, 1989 to Westhead, incorporated herein by reference.
However, the foregoing attempts in the art have not proven entirely successful. For example, woven drying clothing is limited to the patterns which are provided by a repeatable and stable weave. Unlimited patterns are not feasible. Only limited geometries of grooves may be provided to handle the entrained air. The present invention overcomes these disadvantages and provides greater flexibility and options in determining the geometry of the paper machinery clothing.
Additionally, paper machine clothing is known to experience wear during the papermaking process. Such wear shortens the life of the paper machine clothing, increasing the manufacturing costs. Wear of paper machine clothing is attributed to the temperature extremes which occur in papermaking, the two-way bending which occurs as the clothing passes over drying rolls and reversing rolls, as well as the friction against the rolls and drag across vacuum boxes.
Various attempts have been made in the art to mitigate the wear of paper machine clothing which is inherent in the papermaking process. For example, clothing having stacked warps has become common. In a stacked warp arrangement, oftentimes a first layer of lower, or machine-contacting, warp threads is provided. Also, a second layer of upper, or paper-contacting, warps is provided. The two layers of warps are interwoven by weft yarns. The lower layer of warps may be of larger diameter to provide stability and wear-resistance. The upper layer of warps may be of finer diameter to provide a finer surface which provides more consistent and uniform support for the paper web. An example of stacked warps is found in U.S. Pat. No. 5,114,777, issued to Gaisser.
Yet another attempt in the art is to provide warp and/or weft yarns of noncircular cross-section. Particularly, the yarns of the paper machine clothing may be rectangular, having a greater dimension in the width direction than in the height or Z-direction. This geometry provides more area in contact with the papermaking machinery, thus reducing the contact stresses at any particular point in the yarn. Additionally, noncircular, or rectangular, shaped yarns provide the benefit that more area is presented to the paper side of the paper machine clothing as well. By presenting more area to the paper machine side of the clothing, more contact against the drying cylinders occurs. By providing more contact of the paper against the drying cylinder, more rapid and uniform drying of the paper web is possible.
Accordingly, the art has shown considerable need for a fabric which presents high contact area to the paper to be dried thereupon. Further, there is a need for a papermaking fabric having such high contact area without sacrificing permeability. Finally, there is a need for such a fabric which provides relatively uniform pressure against all regions of the paper to be dried thereupon.
One of skill will recognize that the problems of wear of the paper machine clothing is not limited to production of conventional or hard grades of paper. Such wear also occurs when producing tissue and corrugated grades of paper as well.
However, foregoing attempts to reduce wear of the paper machine clothing have not been entirely successful. For example, stacked warp paper machine fabrics are more expensive than single layer fabrics. Also, such fabrics are prone to sleaziness. Rectangular-shaped warps are not amenable to all types of weaves, particularly, high open area weaves which may be desirable for certain types of papermaking, such as through air drying, useful for making tissue paper. Rectangular cross section filaments for a dryer fabric are illustrated in Statutory Invention Registration H1073, published Jul. 7, 1992 in the name of Hsu, incorporated herein by reference.
The art has also shown considerable need for a way to reduce the wear of paper machine clothing, without constraining the type of weave or type of yarns used for the paper machine clothing. Moreover, there is considerable need in the art for a way to reduce wear that is applicable to any type of clothing, including forming wires, or weave. Further, it would be desirable that such a way to reduce the wear of paper machine clothing reduce the sleaziness of the clothing without affecting its paper-contacting surface.